1. Field of the Invention
The present invention relates to a pressure-sensitive, electrically conductive composite sheet, and more particularly to a pressure-sensitive, electrically conductive composite sheet in which a barrier layer does not slip, the pressure sensitivity can be selected as required, and which exhibits a large change in resistance upon compression.
2. Description of the Prior Art:
Electrically conductive elastomers obtained by dispersing electrically conductive elastomers obtained by dispersing electrically conductive particles in elastic, high molecular weight materials are used conventionally for electronic parts such as rubber switches. When such an electrically conductive elastomer is placed directly onto the surface of an electrode, however, an electric current flows when the electrically conductive elastomer is simply touched, making it difficult to obtain the switching function. Therefore a thin electrically insulating porous film is usually inserted between the electrically conductive elastomer and the electrode so that, when the electrically conductive elastomer is locally compressed, it protrudes through pores in the film over the area in which the pressure is exerted, and comes into contact with the electrode to form a circuit and provide the switching function.
However, this pressure-sensitive, electrically conductive mechanism utilizing a porous film has the following defects.
(a) During assembly, if the porous film slips even slightly, the circuit is not formed when the electrically conductive elastomer is compressed; i.e., it fails to exhibit its switching function. Further, when a porous film is employed, the through holes in it must be in agreement with the positions of the contacts of the key board, as disclosed in Japanese Patent Laid-Open No. 74875/1977. PA0 (b) The porous film is often attached to the electrode by an adhesive so that it will not slip, but the surface of the pores could be covered by the adhesive, which would impair the electrical conductivity. Or else, the porous film could be attached in the wrong position, which would require laborious work in a subsequent step for correction. PA0 (1) The electirc current does not flow in the depthwise direction of the sheet, but only in the lengthwise direction of the sheet. Therefore, limitations are imposed on such electrodes. PA0 (2) The sheet does not exhibit elasticity but has a large stiffness. Therefore, the sheet does provide a uniform surface contact upon compression so that variations in the pressure-sensitive characteristics depend upon the position at which it is pressed.
To remove these defects, an electrically conductive composite sheet has been proposed in which an electrically nonconductive woven fabric is provided on one surface of an electrically conductive sheet, as disclosed in Japanese Patent Laid-Open No. 124650/1980. It is, however, difficult to precisely maintain the distance between the electrode and the woven fabric, or the sheet containing the woven fabric, and satisfactory pressure-sensitive characteristics are not necessarily obtained.
There are also methods according to which reduced quantities of electrically conductive particles are added, or the distance between the electrically conductive particles is increased by the application of an external mechanical force, to impart a pressure-sensitive property. A sheet obtained by such a method, however, exhibits only a small change in resistance upon compression, so that it requires a large compression force, and thus is not suitable for use as a switching element.
Japanese Patent Laid-Open No. 147772/1978 discloses a method of imparting pressure sensitivity by subjecting an electrically conductive magnetic material to the action of a magnetic field, so that the resultant magnetic properties are distributed nonuniformly. This method, however, requires a special manufacturing method and complicated molding steps, and a sheet obtained by this method does not necessarily have a satisfactory durability.
A sheet has also been proposed according to which protuberances made of an electrically insulating material are formed integrally on a plastic sheet which is coated with electrically conductive paint. The electrically conductive composite sheet of this construction, however, has the following defects, and does not exhibit satisfactory pressure-sensitive characteristics.